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More NP-complete Problems

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# More NP-complete Problems - PowerPoint PPT Presentation

More NP-complete Problems. Theorem:. (proven in previous class). If: Language is NP-complete Language is in NP is polynomial time reducible to. Then: is NP-complete. Using the previous theorem, we will prove that 2 problems are NP-complete:. Vertex-Cover.

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### More NP-complete Problems

Prof. Busch - LSU

Theorem:

(proven in previous class)

If: Language is NP-complete

Language is in NP

is polynomial time reducible to

Then: is NP-complete

Prof. Busch - LSU

Using the previous theorem,

we will prove that 2 problems

are NP-complete:

Vertex-Cover

Hamiltonian-Path

Prof. Busch - LSU

Vertex Cover

Vertex cover of a graph

is a subset of nodes such that every edge

in the graph touches one node in

Example:

S = red nodes

Prof. Busch - LSU

Size of vertex-cover

is the number of nodes in the cover

Example:

|S|=4

Prof. Busch - LSU

Corresponding language:

VERTEX-COVER = { :

graph contains a vertex cover

of size }

Example:

Prof. Busch - LSU

VERTEX-COVER is NP-complete

Theorem:

Proof:

1.VERTEX-COVER is in NP

Can be easily proven

2. We will reduce in polynomial time

3CNF-SAT to VERTEX-COVER

(NP-complete)

Prof. Busch - LSU

Let be a 3CNF formula

with variables

and clauses

Example:

Clause 2

Clause 3

Clause 1

Prof. Busch - LSU

Formula can be converted

to a graph such that:

is satisfied

if and only if

Contains a vertex cover

of size

Prof. Busch - LSU

Clause 2

Clause 3

Clause 1

Variable Gadgets

nodes

Clause Gadgets

nodes

Clause 2

Clause 3

Clause 1

Prof. Busch - LSU

Clause 2

Clause 3

Clause 1

Clause 2

Clause 3

Clause 1

Prof. Busch - LSU

First direction in proof:

If is satisfied,

then contains a vertex cover of size

Prof. Busch - LSU

Example:

Satisfying assignment

We will show that contains

a vertex cover of size

Prof. Busch - LSU

Select one satisfying literal in each clause gadget

and include the remaining literals in the cover

Prof. Busch - LSU

This is a vertex cover since every edge is

adjacent to a chosen node

Prof. Busch - LSU

Explanation for general case:

Edges in variable gadgets

are incident to at least one node in cover

Prof. Busch - LSU

Edges in clause gadgets

are incident to at least one node in cover,

since two nodes are chosen in a clause gadget

Prof. Busch - LSU

Every edge connecting variable gadgets

and clause gadgets is one of three types:

Type 1

Type 2

Type 3

All adjacent to nodes in cover

Prof. Busch - LSU

Second direction of proof:

If graph contains a vertex-cover

of size

then formula is satisfiable

Prof. Busch - LSU

Example:

Prof. Busch - LSU

To include “internal’’ edges to gadgets,

and satisfy

exactly one literal in each variable gadget is chosen

chosen out of

exactly two nodes in each clause gadget is chosen

chosen out of

Prof. Busch - LSU

For the variable assignment choose the

literals in the cover from variable gadgets

Prof. Busch - LSU

is satisfied with

since the respective literals satisfy the clauses

Prof. Busch - LSU

HAMILTONIAN-PATH

is NP-complete

Theorem:

Proof:

1.HAMILTONIAN-PATH is in NP

Can be easily proven

2. We will reduce in polynomial time

3CNF-SAT to HAMILTONIAN-PATH

(NP-complete)

Prof. Busch - LSU

Gadget for

variable

the directions

change

Prof. Busch - LSU

Gadget for

variable

Prof. Busch - LSU